A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative to the traditional internal-combustion engine used in modern vehicles.
One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell typically includes three basic components: a cathode, an anode, and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane electrode assembly (MEA). The MEA is often disposed between porous diffusion media (DM) and fuel cell plates which facilitate a delivery of gaseous reactants, typically hydrogen and oxygen, for an electrochemical fuel cell reaction. Individual fuel cells can be stacked together in series to form a fuel cell stack capable of power a hydrogen fuel cell powered vehicle. An exemplary fuel cell stack is described in Assignee's co-pending U.S. patent application Ser. No. 11/622,492, the entire disclosure of which is hereby incorporated herein by reference.
As shown in FIG. 1, a known fuel cell stack 2 includes a plurality of fuel cells 4 disposed between a dry end unit 6 and a wet end unit 8. Each of the fuel cells 4 includes an MEA disposed between porous DM and a pair of fuel cell plates such as bipolar plates. The dry end unit 6 is generally disposed at a top or “dry end” of the known fuel cell stack 2, which is vertically oriented. The wet end unit 8 is disposed at a bottom or “wet end” of the known fuel cell stack 2. A first end of each of the fuel cells 4 has an inlet aperture formed in a header thereof for delivery of a gaseous reactant such as hydrogen or air to the fuel cells 4. The first end of the fuel cells 4 also has an outlet aperture formed in a header thereof for exhausting of an other gaseous reactant and reaction products such as water from the fuel cells 4. The inlet apertures cooperate to form an inlet manifold 10 of the known fuel cell stack 2. The outlet apertures cooperate to form an outlet manifold 12 of the known fuel cell stack 2. The fuel cells 4 may also have coolant apertures at the first end that cooperate to form a coolant manifold 11 for delivery or exhaust of coolant fluid from the fuel cells 4. It should be understood that a second end (not shown) of the fuel cell stack 2 also has apertures forming manifolds for exhaust of the gaseous reactant, delivery of the other gaseous reactant, and one of delivery or exhaust of the coolant fluid during operation of the fuel cell stack 2.
It has been observed that a temperature profile of the known fuel cell stack 2 after a shut-down operation includes lower temperatures adjacent the dry end unit 6 and the wet end unit 8. The temperature profile is due largely to thermal conduction from the known fuel cell stack at the dry end unit 6 and the wet end unit 8. The temperature profile causes a buoyancy driven free convection 14 within the inlet manifold 10 and the outlet manifold 12. The temperature profile and the buoyancy driven free convection 14 can lead to water evaporation and movement from warmer to cooler regions in the inlet manifold 10 and the outlet manifold 12, and condensation in the cooler regions. A collection of water at the cooler dry end of the known fuel cell stack 2 can lead to ice formation and blockage of fluid ports of the fuel cells 4 under freezing ambient conditions, and related issues during a start-up of the known fuel cell stack 2.
There is a continuing need for a device to minimize water accumulation at a dry end of the fuel cell stack transported by buoyancy driven convective flows inside the manifold of the fuel cell stack. Desirably, the device reduces a risk of reactant tunnel blockage due to ice formation during a shut-down of the fuel cell stack under freezing ambient conditions, and minimizes related issues during a start-up of the fuel cell stack.